This invention relates to a pole-supporting structure in a rotating D.C. electric motor or generator (hereinafter referred to for simplicity as "D.C. motors").
It is known to construct a D.C. motor stator by bolting pole members to the interior of a solid tube of mild steel, the tube acting both as a magnetic path and as a physical pole support and housing. The tube is closed by end members which also mount bearings for the rotor. This design approach gives a motor which has a relatively small number of parts and is cheap to manufacture. It is also relatively simple to fabricate, although at least the end faces of the tube and the pole mounting points must be machined accurately to obtain accurate location of the poles relative to the rotor.
This type of motor, however, is electrically inefficient because of eddy currents induced in the tube when the D.C. supply is derived from a rectified A.C. supply.
It is also well known to mount the poles on a structure formed of laminations stacked axially of the motor and clamped together. This greatly reduces the energy losses due to eddy currents, but at the disadvantage of greatly increasing the complexity of the structure and the required amount of manufacturing work, and thus considerably increasing cost. Among other disadvantages of stacked-lamination stators are (1) the joints between adjacent laminations are exposed along the length of the machine and may attract water or other corrosive liquids, resulting in bursting apart of the lamination stack; (2) the length of the stator is substantially greater than the length of the rotor due to the presence of end frame clamping devices to hold the lamination stack together; (3) the thickness of each lamination is substantially constant and it is not possible to optimise the lamination thickness along the magnetic flux path; and (4) even with careful geometric design and layout of lamination shapes, there is unavoidably a large degree of waste in punching the laminations from sheet or strip.
One object of the present invention is therefore, in general terms, to provide a pole-supporting structure which combines the benefits of the two types of structure discussed above.